1. Field of the Invention
The present invention relates to an air insulated high frequency filter suitable for use in radio devices.
2. Discussion of Related Art
Radiotelephone manufacturers typically use individual RF filters bought from their manufacturer as separate components already tuned to specifications. Filters based on quarter-wave resonators, such as air insulated rod resonators, helix resonators and ceramic resonators, are thus ready made assemblies encapsulated by the manufacturer in a metal case having solder ears to be fixed on a printed board and connectors for the electrical interface. Even from the beginning of the design, a radiotelephone manufacturer takes into account the mounting of filters so that within the radiotelephone frame, which is usually a structure containing compartments and made by casting or possibly by pressing and bending sheet metal, an compartment is usually allocated for the high frequency filters, such as the duplexer. The frame can also be made of plastics or similar material, which at least partially is coated with a conducting material. Besides the filters, there are separate compartments for the high frequency sections of the transmitter and receiver paths, for the logic and for the synthesizers.
The printed boards located in the compartments are shielded with metallic RF covers at the upper side and possibly below, while the compartment side walls belonging to the frame provide the sideways shielding. This known compartment structure of the frame is able to prevent mutual interference between different functional blocks, or the so called electromagnetic interference. In the assembly of a radiotelephone the printed boards with components attached to them are placed in the frame, and the boards are electrically interconnected. Thus also ready made encapsulated filters are attached to the printed boards, the filters and the printed circuit boards requiring connectors for mechanical as well as for electrical connection.
It is also known to manufacture filters realized by strip line resonators, directly on the printed board's surface, whereby the desired resonator pattern is etched on the printed board in connection with its manufacture. Good accuracy and reproducibility are advantages of this method.
The commonly used technique of this type has some disadvantages. The compartment structure may well lead to using the available space of the radiotelephone in a way which is not optimal, but where e.g. the encapsulated antenna filters stand in their own compartment with plenty of space around them. From the point of view of high frequency filters it is unnecessary to place them in their own compartments, even covered by possible RF covers, because the manufacturer always supplies the filters with a housing. The encapsulated individual filters require a printed board as a mounting base, from where signals are transmitted through an expensive connector and a cable to the front end of the receiver, and correspondingly a separate connector and cable to supply signals from the transmitter output stage to the duplex filter. Said mounting must further be somehow fixed to the frame compartment. A filter realized with strip line resonators requires an excellent Q factor, which is not easily obtained on a common printed board substrate.